Technical Field
The present invention relates to a friction material, particularly to a clutch friction material to be used for an automobile clutch or an industrial machine clutch configured to allow and prevent transmission of power.
Background Art
A variety of clutches have been used in the field of well-known power transmission systems, particularly in the field of automobile power transmission systems. Recently, even in hybrid electric vehicles (HEVs) using the combination of a gasoline engine and an electric motor, a clutch is mounted between a gearbox input shaft and either an electric motor output shaft or an engine output shaft. This type of clutch is configured to switch a drive mode between a motor drive mode and an engine drive mode. Additionally, the clutch herein used is a wet-type multi-plate clutch capable of stably transmitting power.
In the wet-type multi-plate clutch as described above, oil intervenes between constituent elements of a power transmission path. Therefore, there is a drawback that degradation in efficiency is caused in regeneration, i.e., in using the motor as a power generator for braking. On the other hand, intervention of oil can stabilize power transmission performance and prolong product life, but may have negative effect on environment. Therefore, it has been considered to use, even in the HEVs, a dry-type multi-plate clutch without intervention of oil in the power transmission path.
In well-known dry-type multi-plate clutches, a friction plate has a large friction coefficient. Therefore, chances are that when a dry-type multi-plate clutch is used in an HEV, an excessive torque is abruptly transmitted in an early stage of clutch engagement in switching from the motor drive mode to the engine drive mode. Also, chances are that a torque cannot be stably transmitted. Because of this, chances are that vehicle body vibrations (juddering) attributed to a drivetrain and/or clutch squeaking (noises) occur. These vehicle body vibrations and/or noises are transferred to a driver and a passenger and possibly make them feel uncomfortable. Especially, unlike engine drive vehicles, the HEVs quietly start moving due to the electric motor. Hence, the driver and passenger are more likely to feel the vibrations and/or noises as described above.
It has been known that these vibrations and/or noises are likely to occur when a clutch system is in a relatively unused condition or in a fatigue condition.
Now, Japan Laid-open Patent Application Publication No. 2013-119899 describes that a dry-type multi-plate clutch is used in a driving force transmission device of an HEV. In the construction of Japan Laid-open Patent Application Publication No. 2013-119899, a front cover, accommodating the driving force transmission device, includes outward discharge ports for releasing wear powder produced from the dry-type clutch to the outside. Additionally, water is prevented from intruding into a sealed space through the outwardly discharging ports.
Japan Laid-open Patent Application Publication No. 2013-44357 discloses that a dry-type friction material containing compounded rubber includes holes/grooves formed in/on its surface opposite to its friction surface so as to discharge reactant gas of the compound rubber.
Japan Laid-open Patent Application Publication No. 2004-144301 discloses a friction material with improved wear performance.
Japan Laid-open Patent Application Publication No. 2000-256652 discloses a dry-type friction material that exerts better wear resistance under the conditions of high-speed rotation and high temperature and can maintain a high friction coefficient.
Japan Laid-open Patent Application Publication No. 2000-37797 discloses a method of manufacturing a friction material blank that does not degrade in burst strength, wear rate and swaging hole workability.